[FIX] calendar: fix form view email button giving traceback

After https://github.com/odoo/odoo/pull/99386, clicking on the email button in
calendar.event form view would give a traceback as it tried accessing event_id
on a calendar.event record. This commit fixes this error as the object in the
template already is an instance of calendar.event.

task-3127534

closes odoo/odoo#109549

Signed-off-by: Arnaud Joset <arj@odoo.com>

addons/calendar/data/mail_template_data.xml

@@ -457,9 +457,9 @@
         <p placeholder="Enter your message here"><br/></p>
 
     </div>
-    <t t-if="object.event_id.user_id.signature">
+    <t t-if="object.user_id.signature">
         <br />
-        <t t-out="object.event_id.user_id.signature or ''">--<br/>Mitchell Admin</t>
+        <t t-out="object.user_id.signature or ''">--<br/>Mitchell Admin</t>
     </t>
 </div>
             </field>